1. Field of the Invention
The field of the invention is related to photovoltaic cells. In particular, the field of the invention is directed to coating photovoltaic cells.
2. Description of the Related Technology
Energy is defined as the ability to do work, and while viable energy is all around us, the ongoing challenge of mankind is to develop ways to harvest this energy. Alternative energy describes energy sources that do not burn fossil fuels in order to generate usable energy. Renewable energy is energy which comes from natural resources such as sunlight, wind, rain, tides, and geothermal heat, which are renewable or naturally replenished. This century especially has led to an increased interest in the development of both alternative and renewable energy sources, driven by the need for environmentally friendly energy sources. The frequently debated and highly controversial prospect of global warming serving as a threat to society is often the hot topic among scientists, engineers, and environmentalists across the globe. With such discussions, the focus often shifts to developing new technologies and investing in current renewable energy resources. Means of harnessing renewable energy include wind, tide, geothermal, hydroelectric, biomass, and solar energies, by the year 2050, the earth will require 14 Terawatts of energy per year to power the planet, and this demand will increase to 33 terawatts by the year 2100.
Sources of renewable energy are important in the today's economic and environmental climate. Providing it as cheaply as possible is an important factor in getting such sources of energy adopted.
Solar energy shows the most promising potential as a renewable energy source. Over 50% of the incoming solar energy that reaches the earth's atmosphere can be absorbed by land. Solar energy can realistically generate up to 600 terawatts annually, and shows the most promising potential as a renewable energy source. This is in comparison to wind, tide, geothermal, hydroelectric, and biomass energy sources, which have lower energy production rates. The amount of energy the sun currently produces is 35,000 times more than the amount of energy our planet consumes. Solar energy is easily harvested for practical purposes, such as the generation of electricity to power homes, buildings, and even entire cities. Researchers are currently developing solar cells to power automobiles and aircraft. Countries with an abundance of sunlight and a population currently without electricity represent the fastest growing market for solar energy. Researchers are continually looking for ways to develop new solar technologies that have the capability of harvesting sunlight into usable energy.
Photovoltaic (PV) cells provide a way to harvest solar energy. When light shines on a PV cell, it may be reflected, absorbed, or pass right through. But only the absorbed light generates electricity. The energy of the absorbed light is transferred to electrons in the atoms of the PV cell semiconductor material. With their newfound energy, these electrons escape from their normal positions in the atoms and become part of the electrical flow, or current, in an electrical circuit. A special electrical property of the PV cell—what is called a “built-in electric field”—provides the force, or voltage, needed to drive the current through an external load, such as a light bulb. Crystalline silicon PV cells are the most common photovoltaic cells in use today. They are also the earliest successful PV devices. Therefore, crystalline silicon solar cells provide a good example of typical PV cell functionality.
As FIG. 1 shows, a typical conventional photovoltaic cell 10 includes an aluminum base layer, an N-layer, an I-layer and a P-layer. A layer of transparent conductive oxide is attached to the P-layer, and a protective layer of transparent glass is provided at the top of the cell for admitting sunlight while protecting the internal components of the cell.
An important factor regarding photovoltaic cells is increasing their efficiency. One major obstacle that currently prevents maximum level efficiency in photovoltaic solar cells is reflection loss. To decrease reflection loss, most manufacturers etch the surface of a solar cell to roughen it. However, today's technology is rapidly expanding in pursuit of a more dependable way to decrease reflectivity in solar cells, particularly PV or photovoltaic cells.
A recent study, published in the Feb. 14, 2010 issue of Nature Materials, performed by researchers at California Institute of Technology (Caltech) demonstrated that Silicon nanowire arrays serve as a promising application for replacement of traditional silicon wafers in photovoltaic (solar) cells. Initial observations revealed that, “In order for Si wire arrays to achieve maximum absorption over the relevant wavelengths and incidence angles of solar illumination, the reflectivity of the Si surfaces must be reduced, and the light passing between the wires must be randomized.” The researchers showed that silicon nanowires impregnated with Al2O3 particles, then coated with the antireflective material SiN, embedded in the clear, non-toxic polymer Polydimethylsiloxane (PDMS) illustrates enhanced absorption in photovoltaic cells. The two features of the experiment that allowed for an increase in absorption enhancement and collection efficiency can be attributed to the SiN reflective coating and Al2O3 particles.
Another study released on Nov. 5, 2008 done by researchers from Rensselaer Polytechnic Institute showcased the development of an anti-reflective coating that allows solar panels to absorb sunlight from almost any angle in the solar spectrum. According to Shawn-Yu Lin, leader of the team and Professor of Physics at Rensselaer, “This new anti-reflective coating consists of seven silicon layers positioned one on top of the other which makes the sunlight bend, and at the same time enhance the anti-reflective properties. The light that should be reflected is now captured thanks to these seven layers, which measure 50 nanometers to 100 nanometers. These anti-reflective layers who perform like a forest which captures the light between the trees are made of silicon dioxide and titanium dioxide nanorods.” The coating is made of silicon dioxide and titanium dioxide nanorods positioned at an oblique angle. The downsides to their experimentation? “The anti-reflective coating requires multi-layer coatings which are slightly more complex than the typical single-layer ones,” Lin told CNN. “The economics of modern day solar cells depends largely on the thickness of the substrate and the processing cost for making the junctions. Our coating has a thickness of 0.7 to 1 micrometer. The additional cost for incorporating our antireflective coating would be two to four percent of that for existing solar cells,” Lin said. Besides the additional cost, the team also discovered that the nanorods they utilized are very fragile and unstable and are investigating new ways to add strength to them. Thirdly, as Professor Darren Bagnall from the Nanoscale Systems Integration Group at the UK's University of Southampton noted in response to the experiment's findings, “It's really only going to deliver maybe four or five percent more power from the solar cell.” Bagnall also noted that the experiment tested the amount of light that's transmitted to the device, not the efficiency of the device itself. Finally, Jeremy Leggett, founder of Solar Century, a leading UK solar-energy company, was cautiously optimistic, stating, “I've seen this before from other university research groups and there is a world of difference between nice results in the lab and commercially viable products.”
A third study released on Feb. 9, 2009 on physicsworld.com was performed by researchers at the AMOLF institute in Eindhoven, Netherlands. Researchers there developed an antireflective coating modeled after the nanostructure of a moth's eye, a bioengineering concept known as biomimicry. Researchers used gallium phosphate (GaP) nanorods on top of a GaP substrate, then measured reflection and transmission simultaneously, creating a metamaterial with optical properties that change gradually as a function of distance. According to research team leader and professor Gomez Rivas, “We showed for the first time that light transmission was dominant, with only a minor part of the [reduced] reflection linked with scattering losses and absorption.”
Final conclusions can be drawn from the above experimentations that an antireflective coating positioned on photovoltaic solar panels reduces the amount of light that presently escapes the cell, optimizing the amount of wavelength light that can be absorbed by the solar cells and thus converted into energy. The loss of light (referred to as reflection loss) from photovoltaic cells currently account for 75% of the light initially absorbed by the photovoltaic device. Antireflective materials reduce the amount of light in forms of both heat and energy that outflow from solar cells. This allows for maximum absorption of light and thus enhances efficiency and practical performance of photovoltaic cells.
Therefore, there is a need in the field to provide means for increasing the efficiency of photovoltaic cells.